The most accurate result of the investigation is:
The marble released from a distance of 15 centimeters will have the most potential energy transferred into kinetic energy, causing the marble to travel the farthest.
This is based on the fact that the potential energy stored in the slingshot increases with the distance pulled back (the greater the pull back distance, the more elastic potential energy is stored). Consequently, when released, this greater potential energy is converted into kinetic energy, allowing the marble to travel farther.